1. Field Of The Invention
This invention relates to improvements in plastic conveyor belts of the type having a plurality of links or modules connected together to form a continuous belt with connecting rods or pins, and in particular to a unique improvement for capturing the connecting rods and an innovative construction of different modules which are assembled to make the conveyor.
2. Background And Prior Art
The art on plastic conveyor belts utilizing modules with inter-fitting link ends is well known and well worked. In such belts a pivot rod connects the interfitting link ends so that the modules can be assembled with the rods to form a continuous belt. The rods permit angular rotation between adjacent modules as required when the belt goes around sprockets or rolls on the conveyors while at the same time the rods connect and transmit the forces between the adjacent modules. In practice these connecting rods are subject to large forces. Due to the broad range of application in which plastic conveyors are commonly used, the forces and the reaction of the rods to these forces is many times unpredictable. It is of utmost importance that the rods that connect the modules be positively captured within the conveyor belt assembly. Failure to accomplish such results in numerous problems, not the least of which is the belt actually falling apart in use. Other problems include interference between partially exposed rods and the surrounding conveyor structure. Additionally, as a practical matter the rods must be easily insertable and removable from the belt, as such is normally required during belt assembly, belt installation or belt repair. Furthermore, it is desirable to accomplish this without the use of any special equipment or tool. Such is particularly important when considering field installation and repair since special tools represent both added costs and inconvenience to the user.
Because of the significant problems that loose conveyor pins have caused, numerous methods have been used to capture the pivot rods connecting the links in plastic conveyor belts. Such methods include forming heads on the ends of the rods, but these heads can be knocked off and they must be removed for replacement of the rods. The heads have been provided by melting the ends of the rod to provide enlarged ends or heads which are larger in diameter than the rod hole and thereby prevent the rod from moving inwardly through the belt, i.e., the enlarged heads provides means to capture the rods. However, there are numerous problems with this solution to the problem of capturing the rods. First, special equipment is normally required to thermally form the heads. Secondly, the heads are exposed on the edges of the belt in a vulnerable location since any protuberance on a conveyor can either wear or knock the heads of the rods thus allowing the rods to fall out of the belt. Thirdly, there is the problem of the Poisson effect, i.e., when a material undergoes a change in dimension due to an elastic deformation along one axis an opposite change in dimension or deformation occurs along a perpendicular axis. The amount of this opposite deformation is determined by Poisson's ratio. When the conveyor belt is in operation the rods are subjected to compressive forces perpendicular to the axis of the rod. These compressive forces can deform the rod making the diameter of the rod smaller in accordance with the theory of elasticity. In accordance with the Poisson effect the rod then elongates along its axis; in effect, the rod becomes longer than its original length. This in turn causes the rod to protrude further beyond the edge of the belt causing further problems of interference with conveyor structure which can result in significant belt damage and possible down time.
Another way of capturing the rod within the belt is to form a circumferential bead the internal diameter of which is less than the diameter of the rod, the beads being formed at the ends of the rod holes. Such is shown in U.S. Pat. No. 2,911,091 granted Nov. 3, 1959. However, such capturing of the rod is more or less permanent which doesn't take into conditions the need for disassembly and repair of the belt from time to time. Another solution to the problem of capturing a rod end is disclosed in U.S. Pat. No. 3,726,569 granted Apr. 10, 1973, in which the end of the rod hole and the outermost link end are plugged to prevent the rod from escaping from the belt. See also, U.S. Pat. No. 4,709,807 granted Dec. 1, 1987. However, such plugs can be inadequate due to the rod elongation force caused by Poisson's effect mentioned above and threaded plugs can cause stress risers and possible failure, in addition to extra manufacturing time and the cost of threading both the plug and the hole.
Another known method of capturing the rod is a snap-fitting end cap installed axially into the module rod hole or transversely into the module blocking off the rod hole. However, the general design requirement for snap-fit assembly as currently known requires that the plug or end cap be flexible so that its snap projection can deform during installation. This flexibility, which is normally accomplished by placing the snap fit projection at the ends of two flexible arms, also weakens the plug or cap and reduces its ability to resist rod elongation forces. Further, end caps which are installed axially into the rod hole place the entire rod elongation force caused by the Poisson effect on relatively small snap-fit projections. This results in the rods "popping" the end caps off of the end modules.
There is a need in the art for an improved arrangement for capturing the rods inter-linking modules of modular plastic conveyors.
Furthermore, one common design of plastic conveyors includes modules having a plurality of spaced interfitting link ends with intermediate members connecting opposed link ends, and one or more transverse members between the link ends connecting the intermediate members. See, for example U.S. Pat. No. 4,557,374 granted Dec. 10, 1985; 4,556,142 granted Dec. 3, 1985; 4,438,838 granted Mar. 27, 1984; 4,159,763 granted July 3, 1979; 4,080,842 granted Mar. 28, 1978; 3,870,141 granted Mar. 11, 1975; and German Patent No. 113,669 granted Nov. 19, 1899. In one common design the link ends are all of substantially the same width, the modules are reversible and they are assembled in a "brick-lay" pattern so that the ends of one row of modules don't align with the ends of the adjacent row of modules. It is well known that this contributes further strength. In the known design, the sprocket teeth which drive the belt are either necessarily small, or enlarged sprocket tooth cavities (for permitting larger sprocket teeth) are formed between two transverse members and two intermediate members. It is desirable to have a modular design which permits large sprocket teeth, both in length and depth since sprocket tooth size is a key factor in determining both the wear resistance and strength of the sprocket. However, a large sprocket tooth cavity results in the placement of a link end between two intermediate members. In such a construction, when the belt made up of such modules is placed under a load as is the normal case in operation, the stress flow of the link ends located in the sprocket tooth cavity area must flow down the link end around the cross member and then to intermediate members connecting to link ends on the opposite side of the modules. With this construction the link end which is located between the two intermediate members cannot carry the same load as link ends on opposite sides of the module that are directly connected by an intermediate member.
In order to reduce manufacturing cost which is related to the costs of materials used in the belt, which in turn is related to the thickness of the link ends and to the molding cycle, it is desirable to maximize the strength of the belt and module versus the weight and to maximize the strength versus maximum thickness ratios since these two values are critical in determining the manufacturing cost.
For assembly of a belt in a brick-lay pattern there is need to control the centers where the modules occur to assure that all sprocket tooth cavities line up. If brick-laying is started improperly, the belt must be pulled apart and reassembled. There is a need in the art for a modular belt which accomplishes the foregoing desirable results.